Influenza has probably been around as long as human beings have inhabited the earth. Hippocrates first described an outbreak of a flu-like illness in 412 BC (Barry, 2005).
In modern times, the most serious and deadly influenza pandemic occurred in 1918 when nearly 50 million people died worldwide from the “Spanish flu” or “La Grippe.” More deaths occurred during this pandemic than occurred in World War I, or the Great War (in which 20 to 40 million people died).
Many public health officials still consider this pandemic of 1918 to be an ominous warning for today because many questions about the epidemic (such as its origins, unusual epidemiologic features, and its pathogenicity) remain unanswered (Taubenberger & Morens, 2006).
In 1957 the “Asian flu” (H2N2) and in 1968 the “Hong Kong flu” (H3N2) infected and killed even more people. Both strains remain a concern today. Influenza (or the “flu”) is a highly contagious respiratory disease that can cause significant illness and death in both children and adults. Caused by the influenza virus, it
- strikes (on average) 5% to 20% of the population in the United States each year.
- occurs more frequently in late fall and winter months as individuals spend more time with each other in close contact and in enclosed spaces (Dolin, 2017).
More than 200,000 people are hospitalized in the United States from flu-related complications and approximately 36,000 people die from flu-related causes (Centers for Disease Control and Prevention [CDC], 2017b). Worldwide, influenza epidemics cause 3 to 5 million cases of severe illness and about 250,000 to 650,000 deaths (WHO, 2017b). Young children, older adults, and those with certain health risks (such as those who are immunocompromised) are at the highest risk for complications.
HOW INFLUENZA CHANGES
According to Michael et al. (2009), influenza viruses continually shift by two basic processes:
1. Antigenic drift—an ongoing process in which mutations occur within the HA or NA genes of the influenza A and B viruses.
- Based on the extent of the mutation, the resulting viruses may or may not be recognized by previously developed antibodies.
- If this drift occurs in a given season, the current vaccine may not be able to provide protection against the disease.
- This is why vaccines have to be updated seasonally.
2. Antigenic shift—a change resulting from the replacement of HA and sometimes the NA with new subtypes that have not been present in humans in a long time.
- This shift can create significant health risks and occurs exclusively in influenza A viruses.
- This type of shift is uncommon and unpredictable, often results in the introduction of a new strain of virus, and can cause worldwide outbreaks of severe disease without warning.